Which item evolves Pikachu into Raichu in the main Pokémon games?

A Thunder Stone evolves Pikachu into Raichu in the main Pokémon games.

Pikachu is an Electric-type Pokémon introduced in the first generation. Unlike many Pokémon, it does not evolve by reaching a particular level. Instead, the player uses a Thunder Stone, an evolutionary item associated with electrical energy.

In the original games, Pikachu evolves into Raichu when the Thunder Stone is used on it. Evolution can be postponed because Pikachu may learn some moves earlier than Raichu, which makes the timing of the item important to some players.

The anime adds a separate storyline in which Ash’s Pikachu chooses not to evolve, so viewers sometimes assume Pikachu cannot evolve or that evolution depends on friendship. Those ideas apply to particular stories or mechanics, but the standard main-series method is the Thunder Stone.